Optical coherence tomography (“OCT”) enables cross-sectional images of biological samples to be obtained with resolution on a scale of several microns to tens of microns, thus allowing for detailed imaging of a tissue microstructure. It has been demonstrated that Fourier-domain OCT (“FD-OCT”) can provide a significantly improved sensitivity over the time-domain OCT, which enables high-speed imaging. For example, FD-OCT has been implemented in two configurations, e.g., spectral-domain OCT (“SD-OCT”) and optical frequency domain imaging (“OFDI”), as described in at least one of International Patent Application PCT/US2004/029148, filed Sep. 8, 2004, U.S. patent application Ser. No. 11/266,779, filed Nov. 2, 2005, and U.S. patent application Ser. No. 10/501,276, filed Jul. 9, 2004. FD-OCT has been shown to have significant potential as a tool for identifying morphological changes in many clinical contexts, including cardiovascular, gastrointestinal, and retinal imaging.
One limitation of conventional OCT systems and methods is that the backscattered light from only one angular range centered at 180 degrees is collected. The same is the case for optical coherence microscopy (“OCM”) systems, in which the array detection can be used to generate en-face two-dimensional images without beam scanning. An example of one such OCM system is shown in FIG. 1, as described in E. Beaurepaire et al., “Full-field optical coherence microscopy,” Optics Letters 23(4): 244-246, 1998. An acquisition of light backscattered from different angles can be implemented using a technique of angular compounding, which may reduce speckle. Speckle generally manifests itself as a checkered pattern within scattering regions of the image, and makes it more difficult to discern subtle reflectance differences in the tissue reflectance.
A method and system for acquiring backscattered light at different incident angles in the context of OCT enabling angular compounding employs path length encoding. The example of such system is shown in FIG. 2, as described in N. Iftimia et al., “Speckle reduction in optical coherence tomography by ‘path length encoded’ angular compounding,” Journal Of Biomedical Optics 8(2): 260-263, 2003. For example, an optical glass can be placed in the imaging beam path, splitting the incident field into two or more beamlets. This optical glass causes a portion of the incident beam (beamlet 2) to experience a greater path length delay than beamlet 1. In addition, beamlet 2 illuminates the sample at a different angle than beamlet 1. As a result, multiple OCT images of the sample (each acquired at a different angle) appear simultaneously on the OCT display. While being amenable to high-speed imaging, these method and system generally do not scale appropriately to a large number of angles, and can involve a tradeoff between the spatial resolution and the number of angles acquired thereby.
Another method and system translates a right angle prism, directing light from the sample arm to different positions on the focusing lens. An example of such system is shown in FIG. 3, as described in M. Bashkansky et al., “Statistics and reduction of speckle in optical coherence tomography,” Optics Letters 25(8): 545-547, 2000. In these method and system, a backscattered light at a narrow angular range centered at 180 degrees is generally collected, but the angle of incidence of the incident beam with respect to the sample normal varies with the position of the prism. Such method and system likely do not provide for (or even allow) a measurement of angular backscattering distributions. The speed at which the images can be acquired may be limited by the speed at which the prism can be translated in an oscillatory manner. In yet another method and system, detection of the OCT signals with four detectors can be performed simultaneously, which enables angular compounding for the speckle reduction. An example of such system is shown in FIG. 4, as described in J. M. Schmitt, “Array detection for speckle reduction in optical coherence microscopy,” Physics In Medicine And Biology 42(7): 1427-1439, 1997. In particular, the reference beam in this system is generally not larger than the incident beam. Thus, this system may not be conducive to measurements of the angular backscattering distributions. Furthermore, while each detector element receives the light backscattered at a different angle, the solid angle subtended by the light collected for a given detector element is contained entirely within that subtended by the incident beam. The detection in this system is performed in the time domain.
In the field of light-scattering spectroscopy, it is known that the angular distributions of backscattered light generally contain information regarding the size distributions of the scattering particles within the tissue. Given the optical resolution limitations of OCT, the ability to derive robust contrast between tissues with subtle differences in reflectance properties may (in certain circumstances) utilize the measurements of the angular distributions of the backscattered light. Depth-resolved angular backscattering measurements using the low-coherence interferometry have been designed for the light-scattering measurements with high angular resolution, as shown in the arrangements of FIGS. 5(a) and 5(b), as described in A. Wax et al., “Measurement of angular distributions by use of low-coherence interferometry for light-scattering spectroscopy,” Optics Letters 26(6): 322-324, 2001, and FIGS. 6(a) and 6(b), as described in J. W. Pyhtila et al., “Determining nuclear morphology using an improved angle-resolved low coherence interferometry system,” Optics Express 15(25): 3474-3484, 2003.
For example, light from a low-coherence source is divided into two arms of a modified Michelson interferometer, one beam being incident on the sample (or a sample arm) and another being incident on a mirror (or a reference arm). A lens placed in the reference arm can be translated in a direction parallel to the mirror face in order to provide the selectivity for different backscattering angles in the former arm. Measurements of interfered light are generally made in either the time domain (using the arrangement shown in FIGS. 5(a) and 5(b)) or the frequency domain (using the arrangement shown in FIGS. 6(a) and 6(b)). These techniques generally do not permit simultaneous measurements of the angular backscattering distributions, and the measurement speed is likely limited by the speed at which the lens can be precisely translated. While optimized for angular, point-sampling, in-situ measurements, angle-resolved LCI in its current implementations may likely be unsuitable for in-vivo clinical imaging.
Accordingly, there is a need to overcome the deficiencies described herein above. Indeed, simultaneously measuring the light that is backscattered from multiple angles in the imaging context of the optical coherence tomography may allow for high levels of speckle reduction and additional forms of image contrast.
Accordingly, there is a need to overcome the deficiencies described herein above.